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Cheiroballistra
= Introduction = The this reconstruction is mostly based on Iriarte's (2000; 2003) interpretation of the cheiroballistra. When discussing the ancient texts I have had to resort to English translations provided by Marsden (1971: 206-233) and Wilkins (1995: 10-33) and comments made by Iriarte (2000; 2003). Analysis of the archaeological finds is based on the numerous publications of Baatz. = Main controversies = Outswinger or inswinger Archaeological finds strongly suggest that the cheiroballistra was an inswinger so I've reconstructed it as such. This issue has already been discussed in detail here. Winched or not winched? There is another long lasting disagreement about general mode of operation of the cheiroballistra. Some scholars (e.g. Marsden, Wilkins) have assumed the cheiroballistra was cocked with a winch. Then there are those (e.g. Baatz, Iriarte, Stevenson) who believe the cheiroballistra was cocked similarly to gastraphetes by pushing the slider back with one's bodyweight alone. It should be noted that Marsden did not have a chance to see any archaelogical remains of cheiroballistra -style ballistas before his early death. Wilkins (1995) has lots of arguments in favor of a winched cheiroballistra in both of his JRMES articles (1995; 2000). First, he states that the cheiroballistra had the same spring diameter as old wooden-framed one-cubit or two-span ballistas and thus had to be cocked with a winch (1995: 39; 2000: 96). However, as Iriarte (2000: 57) points out, both Wilkins (1995: 21) and Marsden (1971: 223-224) increased the inner diameter of the washers given in text (2 dactyls). This quickly led both into problems, the most immediate one being the inadequate length of the levers resting on top of the washers. If the spring diameter is enlarged, the levers - in text 3 dactyls long - would no longer protrude from the washers enough and tightening the spring bundles with a spanner would be impossible. Therefore Wilkins (1995: 24) had to lengthen the levers to 3,5d. Marsden (1971: 222) with his more conservative increase of washer diamater did not strictly have to do that - end result being that his levers did not protude from the washers and as such did not fulfill their purpose. Second, Wilkins (1995: 39; 2000: 94) clearly assumes all pre-cheiroballistra ballistas had winches. This idea is almost entirely based on two statements given by Heron in his Belopoeica (e.g. Marsden 1971: 21, 25). However, as is clear from Marsden's translations Heron is simply describing what the engineers had to do to be able create even stronger ballistas. No amount of text analysis will ever make Heron say that less powerful ballistas cocked in gastraphetes-style (Marsden 1971: 23) were discontinued or that torsion springs themselves - whatever their size - were too powerful to be cocked manually. It is clear that Wilkins places way too much faith in almost irrelevant statements of one ancient author he considers "authoritative" (Wilkins 2000: 94, 99). Another argument in favor of a winched cheiroballistra is it's weight, as Wilkins (1995: 39) points out. The machine can't be too heavy, or the advantages of winchless operation are lost. Another related issue is the centre of gravity of the machine. According to Wilkins Digby Stevenson has reconstructed a gastraphetes-style cheiroballistra which weighed ~12 kilograms (Wilkins 1995: 39; 2000: 97). Iriarte's reconstruction weighs only 9 kilograms (Iriarte 2000: 65). Most of the weigh of the machine comes from the metal parts - or parts thought to be made of metal. As many dimensions of these parts are missing, reconstructions will be highly subjective and will be based on the assumed strain levels the machine. Therefore the weight of winched cheiroballistras cannot be taken as proof against the stomach bow theory. Wilkins also argues - not very convincingly - that because the material for the handle (part of the triggering mechanism at the end of the slider) was said to be of iron, it should be taken as a further proof that cheiroballistra was winched (Wilkins 1995: 16). He argues that usually P.H. did not state which material to use, so unless considerable strain was placed on the handle, other material such as bronze should have been perfectly satisfactory and no mention of iron would have been necessary. This does not make much sense to me, as the material for the little ladder and it's rungs and cross-piece are not given, either, and these are under considerable stress, too (see Marsden 1971: 214-217; Wilkins 1995: 27-28). Also, the weakest link in the handle and slider construction is the wooden slider, not the handle. This is because the handle is attached to the end of the slider and - as we know - wood splits very easily. Wilkins (1995: 16) noticed this and realized that the end of the slider had to be reinforced with metal plating. Finally, as Iriarte (2000: 57) points out, there is archaeological evidence of small ~2 dactyl washers from Ephyra and Elginhaugh. This proves that small ballistas were in fact used. There are basically two ways to discredit these finds. First, one can argue that this kind of small washers were never used widely and these are just isolated cases. This is, however, unlikely due to how archaeological record is formed. Archaeological finds consist almost exclusively of abandoned and/or lost items. In addition, only a small fraction of the items that were lost or abandoned ever reach the archaeological record for a variety of reasons. So, unless were talking of a closed find such as a city overrun by volcanic ashes or covered by a landslide - or perhaps a shipwreck - we're most likely going to encounter common items of little monetary or reuse value. Therefore it's very likely that we're going to find lots of these small washers in the future. The second counterargument is that the washers belonged to "toys", not real weapons. Whether they were toys did not depend on the washer diameter but the performance (as defined here) of the ballistas they belonged to. In a nutshell there is no reason to forcefully interpret the cheiroballistra text to describe a winched weapon or to try discrediting the stomach-bow theory as Wilkins (1995: 38-41) does. Power and performance I found it necessary to write this section because Wilkins seems to consistently confuse power with performance. In this article power refers to either power fed into the system (ballista) or the kinetic energy of the projectile. Performance, on the other hand, refers to the weapon's usability and effectiveness in conflict situations (sieges, battlefield etc.). Wilkins (2000: 94, 96) argues that stomach-bow proponents are not taking into account the "performance requirements" of ancient catapults. Being a winch proponents what he really meant to say is "power requirements". He does not take into account the performance of the weapon as defined here. What is clear, however, is that the cheiroballistra had to be somehow superior to handbows and other missile weapons of the era to make any sense. Unlike Wilkins seems to think, the power is only small part of the equation. Other factors include things like cost of manufacture, ammunition and training, as well as power, accuracy, range, initial velocity and weight of ammunition. One key factor is also the effeciency of energy transfer, meaning how much of the energy put into the torsion springs is transmitted into the bolt as kinetic energy. Being superior to contemporary missile weapons even in one or few of these areas might be enough to justify use of relatively low-powered hand-cocked weapon. Cheiroballistra's only undisputed disadvantages compared to the handbow is it's weight and somewhat slower rate of fire. Cost might be another, but complex handbows made from sinew, wood and horn were not cheap, either. Also, unlike Wilkins (2000: 96) thinks, the weight of the ballista bolt has little to do with it's performance as a weapon. He argues that light bolts (as in 25 or 42 grams) would be useless in war. This is the case only if the bolt's speed is very low similarly to those in Wilkins' own reconstructions (Wilkins 2000: 93): * Cheiroballistra: 47m/s with 100 gram bolt (110 joules). Apparently the pull was around 739 pounds as in Wilkins' earlier tests. * Three-span ballista: 37m/s with 200 gram bolt (136 joules). These are not especially impressive results given the amount of energy fed into the system. As a comparison a few tests from the author and Tim Baker who has done extensive research on performance of traditional handbows: * From Baker (2000: 114-115): ** Average 40 lbs all-wood, straight handbow: 42,09 m/s avg using a 32,5 gram arrow (29 joules) ** Average 50 lbs all-wood, straight handbow: 45,75 m/s avg using a 32,5 gram arrow (34 joules) ** Average 60 lbs all-wood, straight handbow: 49,72 m/s avg using a 32,5 gram arrow (40 joules) ** Average 70 lbs all-wood, straight handbow: 53,99 m/s avg using a 32,5 gram arrow (47 joules) * Author's ~150 lbs crossbow with steel bow: ** 55,7 m/s avg using a 28 gram bolt (44 joules) ** 47,7 m/s avg using a 50 gram bolt (56 joules) * Author's ~300 lbs crossbow with steel bow: ** 54,4 m/s avg using a 52 gram bolt (77 joules) ** 46,5 m/s avg using a 81 gram bolt (88 joules) Note that due to the shorter power stroke crossbows need to have draw weight 2-3 times higher than handbows to achieve same energy storage levels. Also note that the heavier weight handbows could use heavier arrows to increase kinetic energy of the arrows. Handbows used for war pulled 100lbs and more (***reference***), so the kinetic energy of their arrows is pretty close to those of Wilkins cheiroballistra bolts. It seems clear that efficiency (as defined above) of Wilkins' reconstruction is very bad. As weapons of war they would have been nearly useless due to their weight, slow rate of fire and poor range. Light bolts from a properly designed ballista would have been effective, as long as the velocity was high enough. There is evidence that a properly designed Orsova ballista reconstruction with inswinging arms can consistently reach velocities of 90 m/s with ~400 gram ammunition and 5000 pound draw weight. As of 3th August 2010 this reconstruction has not yet even reached it full potential, but still the velocity is nearly twice as high as in Wilkins' cheiroballistra. Increase in velocity alone quadruples the energy of the bolt when compared to Wilkins' reconstruction. There is no reason to think that velocity would be significantly reduced by scaling down the machine, as long as bolt weight is scaled down, too. This means that if simillar initial velocities can be obtained with cheiroballistras cocked by hand they have three main benefits compared to handbows: * More kinetic energy * Longer range * Better penetration due to smaller cross-section of the bolt Many scholars have used archaeological finds of bolts or boltsheads as basis for their ballista missile reconstruction (e.g. Iriarte 2000: 66-68; Wilkins 1995: 45; Wilkins 2000: 95-96). This is dangerous because it tends to limit testing to random bolts originally belonging to random ballistas (or even crossbows). This almost certainly gives false impression of a ballista's capabilities such as range and power. The problem should approached the other way around, by rigorously testing different bolt weights to find the best match between kinetic energy and initial velocity for each individual ballista. If too light bolt is used, the arms have lots of energy left at the end. On the other hand, if too heavy bolt is used, the ballista will transfer energy efficiently into the bolt, but the initial velocity and range of the bolt may be disappointing. = Cheiroballistra parts = Conventions All measurements are in Greek dactyls (1,93cm). Measurements which are derivable from the text are marked in green. Measurements which are subjective and not given in text are marked in red. Case The case is the lower part of the cheiroballistra stock with a female dovetail groove running down it's length. The upper part of the stock, slider, has the male dovetail which allows it to slide on top of the case. Although the description of the case (e.g. Marsden 1971: 213) is relatively clear compared to most other sections in Heron's cheiroballistra, it can still be interpreted in a number of ways. The part describing the location of the projecting block (ΚΘ) is corrupt in all manuscripts and does not make sense as is. A simple solution to this corruption was suggested first by Prou's (1877: 120-121) and later Iriarte's (2000: 48). Both simply substituted ΑΘ with ΛΘ and the text makes perfect sense. While this theory sounds most plausible to me, other explanations have been suggested by Marsden (1971: 218), Wilkins (1995: 11-12), Schneider (1906: 149) and Baatz (1974: 70). The actual purpose of the projecting block has confused pretty much every researcher, as Iriarte (2000: 48) points out. I have interpreted it simply as a support for the little ladder holding the field frames. This is the simplest solution to keep the little ladder, the field frames and the little arch from moving backwards when the weapon is cocked. Of course, some additional ironwork is needed, but much less than without support from the projecting block. As Wilkins (1995: 12) notes, removing wood along ΛΘ and ΑΚ as suggested by Heron (e.g. Marsden 1971: 213) seems silly. It seems clear that Heron is not thinking like a carpenter, who would have simply glued or nailed a piece of wood to bottom of the board ΑΒ and be done with it - as did I. Full CAD drawing of the case below: Slider The slider has a male dovetail corresponding to the female dovetail in the case. Although a relatively simple component, it's exact form is still not clear. There are two competing interpretations for the slider's cross-section: * Most scholars (e.g. Marsden 1971: 218, Wilkins 1995: 11) have reconstructed the slider from two pieces forming a "T" shape. The lower part of this composite construction was the male dovetail to which the upper part was attached. The upper part simply rests on top of the case. * Iriarte (2000: 52) suggests that the slider was made from one piece. These differing interpretations stem from the fact that Heron did not state how wide the female dovetail should be; he only gives it's depth (1d) and length (46d). He also says that the slider should be "about" 2,5d wide and 1,25d high. The "T" proponents take 2,5d to mean the width of the upper (non-dovetail) part of the slider, whereas Iriarte (2000: 52) suggests that the male dovetail itself - being the only part of the slider - was about 2,5d wide. I've personally followed Iriarte's interpretation as it is simpler and requires one to make fewer questionable assumptions. The "about" (see Iriarte 2000: 52) in slider width requires some discussion. If the slider was 2,5d wide, then only 0,5d (or ~1cm) of wood would be left on both sides of the slider. This is not much, but might be enough for durable operation. Nevertheless, I've made the slider slightly narrower (2d). Crescent-shaped piece Little ladder Field frames Triggering mechanism The triggering mechanism in cheiroballistra text is very vaguely described. Fortunately the manuscript diagrams (see Wescher 1867 and Schneider 1906) clarify the text a lot. Regardless, dimensions of the components are lacking; only the length of the incision in the claw is given, as Iriarte (2000: 53) points out. This reconstruction of the trigger is based on Iriarte's (2000) work. Wilkins reconstruction, though commendable, is based on the idea that cheiroballistra had a winch (1995: 14-17). Fork Similarly to the claw, all authors agree upon the general appearance of the fork or double bracket + tenon as Wilkins (1995) calls it. The fork is bored at ΤΥ. The claw is fitted in between the fork so that an axle can be pushed through fork at ΤΥ and claw at Φ, respectively: Claw The claw is well-known from older artillery pieces, so there is little disagreement on it's general form (see Marsden 1971: 219-220; Wilkins 1995: 17; Iriarte 2000: 52-53). The claw has an incision of 1 dactyl long and has a horizontal, round hole at Φ. This hole is used for the claw axle which also goes through holes in the fork. Trigger Similarly to the claw, the general form of the trigger is well know and agreed upon: Handle The interpretation of the handle depends on whether one reconstructs a winched weapon or not. The general form is handle is surely the same as shown below: In the text it's stated clearly that there's a round hole at Δ, at the bottom of the handle. A corresponding hole is bored to the slider (ΓΔ) at ΜΝ. The handle and slider are then attached together with a pin/axle. Also, a rectangular hole is pierced to the slider at Ξ. The location of this second, rectangular hole is only marked into one of the manuscript diagrams - it is located just behind the round hole (ΜΝ) (see Wescher 1867: 127-128). Iriarte (2000: 52-53) interpreted the rectangular hole as a lengthwise, rectangular incision extending forward from the end of the slider. In Iriarte's reconstruction the handle rotated freely up and down, which was necessary to lock and unlock it to it's anchor (a strong nail) at the end of the draw and after release. Wilkins (1995: 16-17) placed the handle horizontally and used it as attachment point for the winch. He made the rectangular hole perpendicular to the stock, which allowed him to make the handle stronger. This suited his winched cheiroballistra scheme better. = Relation of the parts = Category:backup